The present invention relates generally to sputtering systems, and more particularly to sputtering insulating materials in a rotating cylindrical magnetron using a DC reactive sputtering method.
DC reactive sputtering is the process most often used for large area commercial coating applications, such as the application of thermal control coatings to architectural and automobile glazings. In this process, the articles to be coated are passed through a series of in-line vacuum chambers isolated from one another by vacuum locks. This may be referred to as a continuous in-line system or simply a glass coater.
Inside the chambers, a sputtering gas discharge is maintained at a partial vacuum at a pressure of about three millitorr. The sputtering gas comprises a mixture of an inert gas, such as argon, with a small proportion of a reactive gas, such as oxygen, for the formation of oxides.
Each chamber contains one or more cathodes held at a negative potential of about -200 to -1000 volts. The cathodes may be in the form of elongated rectangles, the length of which spans the width of the line of chambers. The cathodes are typically 0.10 to 0.30 meters wide and a meter or greater in length. A layer of material to be sputtered is applied to the surface of the cathodes. This surface layer or material is known as the target or the target material. The reactive gas forms the appropriate compound with the target material.
Ions from the sputtering gas discharge are accelerated into the target and dislodge, or sputter off, atoms of the target material. These atoms, in turn, are deposited on a substrate, such as a glass sheet, passing beneath the target. The atoms react on the substrate with the reactive gas in the discharge to form a thin film.
The architectural glass coating process was made commercially feasible by the development of the magnetically-enhanced, planar magnetron. This magnetron has an array of magnets arranged in the form of a closed loop and mounted in a fixed position behind the target. A magnetic field in the form of a closed loop is thus formed in front of the target plate. The field causes electrons from the discharge to be trapped in the field and travel in a spiral pattern, which creates a more intense ionization and higher sputtering rates. Appropriate water cooling is provided to prevent overheating of the target. The planar magnetron is further described in U.S. Pat. No. 4,166,018.
A disadvantage of the planar magnetron is that the target material is only sputtered in the narrow zone defined by the magnetic field. This creates a "racetrack"-shaped sputtering zone on the target. Thus, a "racetrack"-shaped erosion zone is produced as sputtering occurs. This causes a number of problems. For example, (1) localized high temperature build-up eventually limits the power at which the cathodes can operate, and (2) only about 25 percent of the target material is actually used before the target must be replaced. Another significant problem, affecting uniformity and stability, is the build-up of oxides on the target outside of the erosion zone. This leads to arc discharges which temporarily perturb the gas discharge conditions. The arcing problem is very severe when silicon dioxide is being deposited by reactive sputtering of silicon.
The rotary or rotating cylindrical magnetron was developed to overcome some of the problems inherent in the planar magnetron. The rotating magnetron uses a cylindrical cathode and target. The cathode and target are rotated continually over a magnetic array which defines the sputtering zone. As such, a new portion of the target is continually presented to the sputtering zone which eases the cooling problem, allowing higher operating powers. While this cooling is more effective it is still possible for rotating magnetron cathodes to reach a temperature sufficient to melt low melting point target materials such as tin, lead, or bismuth, particularly at the ends of the sputtering zone. It is at the ends where the power density is highest because of the "turn around" portion of the "racetrack".
The rotation of the cathode and target also ensures that the erosion zone comprises the entire circumference of the cylinder covered by the sputtering zone. This increases target utilization and reduces arcing from the target within the erosion zone. The rotating magnetron is described further in U.S. Pat. Nos. 4,356,073 and 4,422,916, the entire disclosures of which are hereby incorporated by reference.
The rotating magnetrons while solving some problems produced others. These problems include new arcing phenomena, which are particularly troublesome in the DC reactive sputtering of silicon dioxide and similar materials such as aluminum oxide and zirconium oxide. Insulating materials like silicon dioxide are particularly useful to form high quality, precision optical coatings such as multilayer, antireflection coatings and multilayer, enhanced aluminum reflectors. Such coatings would be much more economical to produce if they could be applied by an in-line, DC reactive sputtering process.
The true advantages of a continuous, in-line sputtering process, as far as operating efficiencies are concerned, are only realized if the process can be continuously operated to produce acceptable product. Perturbation of the sputtering conditions due to arcing is especially detrimental to cost effective operation, as any article being coated when an arc occurs will most likely be defective. For instance, the article may be contaminated by debris resulting from the arc, or it may have an area with incorrect film thickness caused by temporary disruption of the discharge conditions. Furthermore, the occurrence of arcs increases with operating time, and eventually reaches a level which requires that the system be shut down for cleaning and maintenance.
By way of example, in one rotating magnetron configuration, arcing from cathode ends and bearing structures while depositing silicon dioxide from a silicon target was experienced less than one hour after sputtering commenced. The occurrence of arcs increased rapidly with operating time, reaching a frequency of about one hundred arcs per minute in less than 2 hours. This caused permanent perturbation of the discharge conditions, requiring that the machine be shut down for maintenance. This rotating magnetron configuration is described in J. Hoffman, "DC Reactive Sputtering Using a Rotating Cylindrical Magnetron", Proceedings of the 32nd Annual Conference of the Society of Vacuum Coaters, pp. 297-300 (1989).
In view of the foregoing, an object of the present invention is to improve the effectiveness of the DC reactive sputtering process for silicon dioxide and other materials, which are highly insulating, when deposited by DC reactive sputtering.
Another object of the present invention is to substantially reduce or eliminate the occurrence of arcs in rotating cylindrical magnetrons.
A further object of the invention is to increase the deposition rate for low melting point target materials.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or will be learned from practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.